There seems to be a "proofing gap" for older, still copyrighted books. Distributed Proofreaders won't get them (copyrighted!) and the publishers lack resources.
I got this explicitly confirmed by a Swedish ebook provider (elib.se). At least one of their publishers (Nordstedt) choses to publish their backlist of quite well established authors as scanned images in PDF only since they lack resources to proof-read the OCR results. New books of the same authors are usually available as mobipocket etc.
/Jens
|